mysql - SQL Select至少有2个订单且其中至少有一个在某区的员工，无嵌套SQL block

Question

我有两个表，'Customers' 和 'Employees' 连接到第三个表 'Orders'

| CustomerID | CDistrict |    | EmployeeID | EName |
|------------|-----------|    |------------|-------|
|      1     |     A     |    |      1     |  Alex |
|      2     |     A     |    |      2     |  Bob  |
|      3     |     B     |    |      3     |  Edd  |
|      4     |     C     |

| OrderID | CustomerID | EmployeeID |
|---------|------------|------------|
|    1    |      1     |      1     |
|    2    |      3     |      1     |
|    3    |      3     |      2     |
|    4    |      5     |      2     |
|    5    |      1     |      3     |

如何选择至少服务过 2 个订单的所有员工的姓名，其中至少有一个来自地区“A”的客户没有嵌套 SQL block ？也就是说，结果应该是“Alex”。

我设法通过嵌套查询做到了这一点，如下所示:

SELECT EName FROM Database.Employees
WHERE 
    EmployeeID IN 
    (SELECT EmployeeID FROM Database.Orders
        GROUP BY EmployeeID
        HAVING COUNT(*) >= 2)
AND
    EmployeeID IN
    (SELECT EmployeeID FROM Database.Orders
        WHERE 
            CustomerID IN
            (SELECT CustomerID FROM Database.Customers
                WHERE CDistrict = 'A'));    

但是，我需要在没有嵌套查询的情况下执行此操作。我选择至少服务过 2 个订单的所有员工的查询如下所示:

SELECT EName FROM Database.Employees, Database.Orders
WHERE 
    Employees.EID = Orders.EID
    GROUP BY EName HAVING COUNT(OrderID) >=2
ORDER BY EName;  

我的查询选择了所有为地区“A”的客户提供服务的员工，如下所示:

SELECT EName FROM Database.Employees, Database.Orders, Database.Customers
WHERE 
    Employees.EID = Orders.EID
    AND
    Orders.CID = Customers.CID
    AND
    CDistrict = 'A';    

但是，当我尝试将这些查询加入到单个查询中时，例如

SELECT EName FROM Database.Employee, Database.Orders, Database.Customers
WHERE 
    Employees.EID = Orders.EID
    AND
    Orders.CID = Customers.CID
    AND
    CDistrict = 'A'
    GROUP BY EName HAVING COUNT(OrderID) >=2;

我变成了 0 个结果。我想，这个查询可能会搜索在“A”区至少服务过 2 个订单的所有员工，但这不是我需要的 - 我需要至少服务过两个订单的员工，其中至少一个是在“A”区。我究竟做错了什么？如何将嵌套查询转换为常规查询？

Answer 1

使用 GROUP BY 和 HAVING:

SELECT o.EmployeeID
FROM Database.Orders o JOIN
     Database.Customers c
     ON o.CustomerId = c.CustomerId
GROUP BY o.EmployeeID
HAVING COUNT(*) >= 2 AND
       SUM(c.CDistrict = 'A') > 0;